In today's information society, great amounts of data are used. HDDs are widely used as data storage media. This is because each HDD has a large storage capacity and is relatively inexpensive. Although the HDD can hold much data and is relatively inexpensive, it has a relatively low access speed.
In comparison with the HDD, the flash memory has indeed a high access speed. It is, however, expensive. If SSDs each having a flash memory are used in place of all HDDs in a file server that has several HDDs, the manufacturing cost of the file server will greatly increase.
In view of this, various high-speed data storage systems have been proposed, in which a plurality of data storage media of different characteristics are combined to attain a large storage capacity and achieve a high access speed.
In a data storage system including a first data storage medium that is expensive and operates at high speed, and a second data storage medium that is inexpensive and operates at low speed, the first data storage medium may be used as a cache for the second data medium. Then, an access to the second data storage media can be apparently faster.
In a data storage system having this configuration, the first data storage medium and the second storage medium are accessed independently in most cases. Some measures must therefore be taken to increase the speed and efficiency of the data transfer between the first and second data storage media.